Pulmonary macrophages play a key role in clearing inhaled foreign material from the alveolar surface. Specific antigens have been identified on these cells in several different species in several laboratories. The significance of these specific antigens has not been determined. Their development is not understood. It is not known whether their presence is a reflection of origin, specialization, or antigen contact. The primary purpose of this research is to study the development and nature of these specific pulmonary macrophage antigens. All macrophages are thought to originate in the bone marrow and migrate via the blood stream as monocytes to tissue locations where they specialize to function in that area. The pulmonary interstitium has been suggested as the site where pulmonary macrophages develop their specialized characteristics. Comparison of cells in this area and on the alveolar surface will be the starting point of these studies. Electron microscopy of lung tissue reacted with marker conjugated specific antibody will be used to identify macrophages with specific antigen on the alveolar surface and in the pulmonary intersititium. Quantitative differences in number of cells with antigen and quantity of antigen per cell in each area will be evaluated. Similarly, blood monocytes and cultured bone marrow macrophages will be studied for the presence of antigen. To determine if development of these antigens are related to environmental antigen exposure, neonatal animals will be studied at birth and at weekly intervals till antigens develop. Animals born and raised in germ-free environments will similarly be studied. When the development of these specific antigens is understood, it may be possible to utilize their presence as markers in studying the responses of pulmonary macrophages to inhalation challeges including those which produce granulomatous lesions. It may be possible to identify "resident" and "incoming" macrophages in a response by the presence or absence of antigen. One may then determine morphologic, histochemical, and functional differences or similarities between these two cell types.